Psoralen plus ultraviolet light (PUVA) therapy is widely used in the treatment of psoriasis and has potential application in the treatment of cancers, such as cutaneous T cell lymphoma. Psoralen photoreaction of cells produces a number of genotoxic effects, such as mutations, mitotic recombination and chromosomal aberrations, which are correlated with carcinogenicity, and has been shown to induce skin tumors in psoriasis patients after long-term treatment. The cellular responses to psoralen damage will be studied at the molecular level; by preparing plasmid DNA containing site-specifically placed psoralen adducts in vitro, and transforming them into yeast cells for in vivo repair. The mutant and recombinant products of the in vivo repair pathways will be isolated an analyzed. 1) Mutant plasmids will be analyzed by DNA swquence analysis to determine the nature of psoralen-induced mutations. The sites of the muations will be compared to the psoralen addition sites to determine whether mutagenesis is targeted to damage sites. 2) Psoalen reacted plasmids will be transformed into cells which have been previously subjected to other DNA damaging treatments, to investigate inducible repair processis and determine whether they are mutagenic. 3) Recombinant plasmids will be analyzed to test for targeting of recombination to DNA damage, the extent of gene comversion and its association with crossing over. 4) The effects of crosslinks and monoadducts of mutation and recombination will be compared.